Chemical compounds which decompose to generate acids when exposed to radiation (referred to herein as “photoacid generators”) in the ultraviolet region of the spectrum (i.e., <300 nm) are the basis for “chemically amplified” deprotection or crosslinking of polymers in chemically amplified photoresists for microelectronics applications. The decomposition products of such photoresists including primarily low molecular weight organic molecules such as isobutylene (from high activation energy photoresists) and acetaldehyde (from low activation energy photoresists), but also decomposition products from the photoacid generator, have been observed in photoresists used in imaging tools (steppers) operating at wavelengths of, for example, 248 nm and 193 nm. Outgassing of such materials can coat and corrode the optics.
Measures have been implemented to limit the effects of outgassing of decomposition products such as, for example, cleaning the optics and/or including sacrificial barriers or filters between the optics and the photoresist coating. However, with the industry trend toward increased resolution at smaller and smaller linewidths of less than 45 nm, and with the development of new tools operating at significantly shorter wavelengths (such as in the extreme ultraviolet (EUV) region at 13.5 nm) and having advanced reflective optics, there is renewed interest in control of outgas sing at the compositional level in a photoresist.
Photoresists have been studied for their contribution to the outgas sing of photoresists during EUV exposure. Pollentier (“Study of EUV Resist Outgassing/Contamination for Device Integration using EUVL Processes,” Pollentier, I., J. Photopolym. Sci. Technol., 2010, vol. 23(5), pp. 605-612) has found, after testing several photoresists for outgassing by residual gas analysis (RGA) in which a sample is exposed in a sealed chamber, and the atmosphere after exposure is analyzed by gas chromatography/mass spectrometry, that several of the primary decomposition products of the photoresists tested include low molecular weight compounds such as benzene and diphenyl sulfide, attributed to decomposition products of the photoacid generator (for example, where the photoacid generator studied was triphenylsulfonium trifluoromethane sulfonate). In particular, volatile decomposition products that include sulfur are of concern as these materials may not be effectively cleaned from the optics, which is further problematic as ionic photoacid generators based on the triphenylsulfonium cation have desirably high sensitivity and provide fast photospeed (<10 mJ/cm2) in photoresists.